1. To make the dough: In a large mixing bowl, combine the sour cream, sugar, butter, vanilla, and egg; mix well. Sift in remaining ingredients and mix until well combined.
2. Cover dough and refrigerate to chill for at least one hour.
3. Meanwhile, combine filling ingredients.
4. Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius) and line two baking sheets with Gefen Parchment Paper. Remove dough from refrigerator and roll out on parchment paper into a 14- by 16-inch (35- by 40-centimeter) rectangle. Smear filling over surface of dough. (I used my fingers to spread the filling evenly over the entire surface.)
5. Roll up cookie dough lengthwise and cut into thin slices, about 1/3 inch (3/4 centimeter) thick. Place onto prepared baking sheets, allowing room for spreading. Bake for 10–12 minutes, or until just beginning to turn slightly golden. Remove from oven to cool.
6. Combine icing ingredients in a small bowl. Transfer to a piping bag (or a bag with a small slit cut off one corner). Drizzle icing over cookies. Allow icing to set before storing the cookies.
If you prefer not to put dairy ingredients in your mixer, you can do the following: Melt the butter in a large pot. Once it cools, mix the dough ingredients in the pot. You’ll have to use a little elbow grease, but the cookies come out just fine!